Boulder scientists develop new index for judging hurricane danger

A photo taken Thursday shows damage in Orient Bay on the French Carribean island of Saint-Martin, after the passage of Hurricane Irma. (Lionel Chamoiseu / Getty Images)

Done

As Hurricane Irma barrels toward south Florida as a Category 5 storm, it's being tracked on a different scale from Boulder, where on Thursday it registered a slightly more modest 4.8 — on a scale of 1 to 10.

James Done is the science lead for the Capacity Center for Climate and Weather Extremes at the National Center for Atmospheric Research. He led the development of its Cyclone Damage Potential index, which quantifies a hurricane's ability to cause destruction.

Done sees the index as being more useful for forecasting damage than the Saffir-Simpson Hurricane Wind Scale, the widely recognized 1-to-5 scale for judging damage based solely on wind speeds.

Hurricane Irma earned its 5 rating on that scale Wednesday with top wind speeds of 185 mph, well above the 157 mph-or-higher benchmark required to be assigned that ranking. Thursday, Irma's top wind speed was 175 mph.

"Whereas that scale only considers peak winds, our scale considers peak winds, but also the size of the storm, the area of damaging winds, and how fast it's moving forward, combined," Done said of the CDP index. "That controls the duration of the damaging winds, and that can really drive up losses," with an extended duration spelling more extensive damage.

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On Wednesday, Hurricane Irma's ranking on NCAR's CDP index was 5.5, before slipping to Thursday's rating of 4.8, which the Engineering for Climate Extremes Partnership labeled in a Twitter post Thursday as still "formidable."

Had the index existed at the time the highly destructive Hurricane Andrew savaged south Florida in 1992, that storm would have merited a ranking of only 2.9. Hurricane Andrew killed 44 people in Florida and caused $25 billion in damage.

As for Irma, Done said, "The factor that's making it stay lower than it otherwise would be (on NCAR's CDP index) is that it's moving at 16 miles mph, forward. But as it approaches south Florida it is forecast to stall, slightly, so this reduction in forward speed will cause the damage potential to increase."

On Done's index, Hurricane Harvey, which proved so destructive to southeast Texas, was rated at 5.2 at the time that it made landfall. Factors that made it so catastrophic included the expansive area the storm covered, and also the fact that it had slowed dramatically when it reached the coast.

"Just after landfall, it really crawled, and came back on itself," Done said.

The NCAR index initially was developed about four years ago, in collaboration with the reinsurance industry, but is not yet available to the general public — something Done believes will change by December.

"We're honing in on what is useful and what works and what doesn't work, in terms of communication. Only at that point would we want to put it out more," Done said.

"My main goal is to make our new scientific understanding of hurricanes available to people who can benefit from it. Whether it's through this index, or through other communication tools, I'm happy."

He applauds the National Hurricane Center for having started issuing storm surge forecasts as part of its reports.

"The whole enterprise is moving in that direction, to get a more holistic view of storm impact," providing information on storm surge, inland flooding and more, Done said. "The Saffir-Simpson scale says nothing about that."

If a storm as destructive as Harvey hit land as a 5.2 on the NCAR scale, and the fearsome Irma on Thursday mustered only a 4.8, what would, for example a 9 on the CDP index look like?

"A value of 9 would be at the high end (Saffir-Simpson) Category 5, a very large storm. The area of damaging winds would be at the top end of the scale, something we rather rarely see in the north Atlantic. And it would be almost stationary," Done said.

"The thing that is killing Irma's scale at the moment is its brisk forward speed."

One thing the NCAR index does not do is factor in the terrain that is to be affected by a storm; how the damage to a heavily built-in, populous environment such as Miami might compare to that from a storm that strikes a rural, sparsely settled coastline.

"To understand what your personal impacts are, you'd want to understand what your exposure is in terms of buildings, and how well built they are, and the social cohesion, and combine that with the index," Done said.

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